Allow checkout_branch to point to a tag in clone_repository method

[antazoey]

When I try to provide a tag to the checkout_branch flag in pygit2.clone_repository(), it fails and is unable to find the reference. It would be nice to be able to checkout a tag right away (like a branch) on clone to clone a specifc release of a package

[amyasnikov]

Bump to this issue, I also faced the inability to clone repository using a tag

[postlund]

Same here. Supporting everything a regular git checkout does would be great, i.e. SHA, branch or tag.